In a typical communications network, also referred to as e.g. a wireless communications network, a wireless communications system, a communications network or a communications system, a device, communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
The device is a device by means of which a subscriber may access services offered by an operator's network and services outside, i.e. external to, the operator's network to which the operators radio access network and core network provide access, e.g. access to the Internet. The device may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network. The devices are enabled to communicate wirelessly with the network. The communication may be performed e.g. between two devices, between devices and a regular (landline) telephone and/or between the devices and a server via the radio access network and possibly one or more core networks and possibly the Internet.
The communications network covers a geographical area which may be seen as divided into cell areas, with each cell area being served by a network node. The network node may be referred to as a base station, e.g. a Radio Base Station (RBS), which in some communications networks is also called evolved NodeB (eNB), NodeB, B node or base station. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site. Each cell is identified by an identity within the local radio area, which identity is broadcast in the cell. The base stations communicate with the devices that are within range of the base stations.
The communications network may further include any additional network nodes suitable to support communication between devices. Such additional network nodes may be e.g. a Radio Network Controller (RNC), a Serving GPRS Support Node (SGSN), a Mobility Management Entity (MME) etc. GPRS is short for General Packet Radio Service.
In such a communication system, data needs to be sent in two directions, i.e. both in uplink (UL) (from the user device to the network node, also denoted reverse link) and in downlink (DL) (from the network node to the user device, also denoted forward link). There are different ways, or schemes, of controlling a two way transmission of data. Such schemes may e.g. be half duplex or full duplex. A half duplex transmission is a transmission in two directions, wherein the transmission is possible only in one direction at a time. In a full duplex transmission, sometimes also referred to as duplex transmission, the transmission may be sent in both directions simultaneously. In order to be able to transmit in both directions, a device and/or a base station must have a duplex scheme. There are two forms of duplex that are commonly used, namely Time-division duplexing (TDD) and Frequency-division duplexing (FDD). Some communication systems use only TDD or only FDD, and some use both TDD and FDD.
In FDD, the simultaneous transmission and reception of signals is achieved using two different frequencies. FDD makes it possible to transmit and receive signals simultaneously in time as the receiver is not tuned to the same frequency as the transmitter. FDD transmissions require a certain duplex distance between the transmitter and receiver frequencies and a duplex gap between transmit and receive bands. Such duplex distance and gap are required to separate uplink and downlink channels to avoid interference between uplink and downlink within the FDD band. It may also be necessary to provide a frequency gap, i.e. guard band, within or immediately adjacent to the FDD downlink and/or uplink frequencies to prevent interference towards other services using adjacent frequency bands.
TDD uses only a single carrier frequency and such transmission scheme shares the channel between transmission and reception, spacing them apart by multiplexing the two signals on a time basis. In TDD data transmissions are effectuated transmitting a burst of data in each direction. TDD requires a guard time or guard interval between transmission and reception to ensure that the transmission and reception, within a TDD channel, do not collide and/or interfere. The guard time must be chosen so as to allow sufficient time for the signals travelling from the transmitter to arrive at the receiver before a transmission in the reverse direction is started at said receiver and thus avoids this receiver being inhibited from receiving the transmission. In some scenarios, the data traffic in the two directions is not balanced. There may be more data traffic travelling in the downlink direction of the communication system than in the uplink direction. This means that, ideally, the capacity should be greater in the downlink direction. Using a TDD system, it is possible to change the downlink to uplink data capacity ratio; it can be adjusted dynamically by changing the number of time slots allocated to each direction.
In Long Term Evolution (LTE), TDD is being deployed in several frequency bands globally. Some of the frequency operating bands have been allocated to several operators without any guard bands in between, e.g. in the 2.6 GHz range in Europe. In the current LTE standard it is implicitly assumed that adjacent carrier frequencies in TDD networks are synchronized in order to substantially reduce the unwanted emissions between the carrier frequencies. However it is not an obvious matter of course that competing operators will agree to synchronize their network.
Today, TDD operation has only been demonstrated in a single-operator environment within an operating band, but multi-operator deployment is expected within few years. As mentioned, synchronization between the networks of different operators can typically not be assumed.
The need for synchronization between different operators also implies that the operators must employ the same uplink-downlink configuration on all frequency carriers in order to avoid interfering operation. Further, any data traffic asymmetry in uplink/downlink is then assumed to be the same for all operators, which need not be true and which poses a serious restriction for TDD in general. Without synchronization (including uplink/downlink alignment), guard bands are needed between the operator frequency blocks/bands in order to ensure low interference between networks. However, this reduces the spectral efficiency. In case a TDD network is operated in a band adjacent to an FDD band, guard bands are always needed (FDD-TDD synchronization is not possible as the uplink/downlink configuration cannot be the same).
Users require that the services provided by the operators have high quality, in particular that the services are provided without disruption and with high data rate. The operators thus face difficulties in satisfying the demanding users, e.g. due to the above synchronization need when trying to provide interference free channels and the limited communication resources when trying to provide high data rates.